In the field of gas sensing and analysis, it is well known that when Pd metal is exposed to hydrogen gas, hydrogen molecules dissociate on the Pd surface and the resulting hydrogen atoms can diffuse into the bulk of the Pd, eventually reaching an equilibrium concentration in the metal. It is therefore possible to measure the gaseous concentration of hydrogen by measuring one or more of the physical properties of Pd that are influenced by dissolved hydrogen.
Methods are to measure the physical expansion of a known length of Pd rod (R. L. Collins, Hydrogen Detector, U.S. Pat. No. 3,559,457), the deflection of a bimetallic strip (P. J. Shaver, Bimetal Strip Hydrogen Gas Detectors, The Rev. of Sci. Instru. 40 [7], 901-5, 1969), or the elongation of a Pd-coated optical fiber (M. A. Butler and D. S. Ginley, Hydrogen Sensing with Pd-Coated Optical Fibers, J. Appl. Phys. 64 [7], 3706-12, 1988).
Another technique is to measure changes in the electrical resistivity of a Pd thin film (P. A. Michaels, Design, Development, and Prototype Fabrication of an Area Hydrogen Detector, Bendix Corporation, Southfield, Mich., 1964, Contract NAS8-5282). Because the resistance changes are small, it is necessary to amplify the signal and, at the same time, to provide temperature compensation. Michaels, therefore, used two rectangular Pd thin films, deposited side-by-side on a glass slide. The film dimensions were such that each strip had a resistance of about 10 .OMEGA.. One strip was covered by a thin layer of silicone resin or Mylar tape, and the other was uncovered. An external circuit was constructed such that the two strips formed the passive and active legs, respectively, of a Wheatstone resistance bridge.
The hydrogen sensor described by Michaels had several limitations. Firstly, the Pd was deposited directly onto the glass substrate, and adhesion thereto was not sufficient for use in a practical device, particularly at high hydrogen concentrations. High concentrations of hydrogen generally lead to appreciable mechanical strain in the Pd film and cause it to peel away from the substrate. Secondly, the two-element bridge required external bridge resistors and cumbersome electronics which would make a practical device more difficult to produce. The external resistors very often are variable resistors so that one can adjust them to bring the bridge into balance at some reference condition (e.g., to give a zero reading when no hydrogen is present). These external circuits are usually cumbersome and a source of noise in the output signal. Furthermore, when the active and passive metallizations are configured as two parallel rectangles, their resistance is generally undesirably low in a sensor of any practical size. This in turn requires either a large current in the bridge or a very sensitive amplifier. Furthermore, the low aspect ratio of the Pd strips (L/W.perspectiveto.10) and their correspondingly low resistance (10.OMEGA.) leads to high power consumption and inefficient amplification, which would make a practical device more difficult to operate.
Finally, the use of polymer films as the passivation layer is incompatible with high temperature or hostile environment applications, severely limiting the usefulness of a practical device.